Human

In fact, it's the hardest creature to deal with, in this world.

It's hard to change their mentality,
It's hard to change their attitude,
It's hard to change their behavior,
It's hard to change their preference,
It's hard to understand them.
It's hard to please them.
It's hard to trust them.

So, in terms of work, I guess people management skill is the hardest thing to acquire - for technical skill, you can get the replacement much easier than a good manager (who, apparently, knows how to manage)

And therefore, in life - is that the reason why human chooses to have a pet? (as it's easier to do the few things I mentioned above on pet, rather than on human)

At least they don't cheat.